tmp.sh

#!/bin/bash

bottle=12.5
empty=0.95
s=6.95
i=1
tabspace="  "
defmax=5
if [ -z $1 ]; then
   max=$defmax
else
   max=$1
fi

while [[ $i -lt  $max ]]; do
   total=`echo "($i * ($bottle + $empty)) + $s" | bc -l`
   if [[ $i -ge 5 ]]; then
      total=`echo "$total * .95"|bc -l`
   fi
   echo "Bottles: $i"
   echo "${tabspace}Total: $total"
   echo -n "${tabspace}Cost per oz: "
   echo "$total / (3 * $i)" | bc -l
   half=`echo $total / 2 | bc -l`
   echo "${tabspace}50%: $half"
   i=$(($i+1))
done

Output:

Bottles: 1
  Total: 20.40
  Cost per oz: 6.80000000000000000000
  50%: 10.20000000000000000000
Bottles: 2
  Total: 33.85
  Cost per oz: 5.64166666666666666666
  50%: 16.92500000000000000000
Bottles: 3
  Total: 47.30
  Cost per oz: 5.25555555555555555555
  50%: 23.65000000000000000000
Bottles: 4
  Total: 60.75
  Cost per oz: 5.06250000000000000000
  50%: 30.37500000000000000000